Ovarian hormones and fasting differentially regulate pituitary receptors for estrogens and gonadotropin releasing hormone in the rabbit female

To investigate the mechanisms by which caloric restriction affects reproductive function in female rabbits, we measured, in animals intact or ovariectomized (OVX) estrogen-primed and fed ad libitum or fasted for 48 h, the adenohypophysial expression of estrogen receptor-alpha (ESR1) and gonadotropin releasing hormone receptor (GnRHR) and the dynamic secretion of LH following GnRH stimulation. Fasting increased the number of GnRHR-immunoreactive (-IR) cells in intact animals, whereas reduced the density of ESR1-IR cells in OVX rabbits. Estrogen priming decreased the number of ESR1-IR cells in fasted and OVX animals. Ovariectomy increased the number of ESR1-IR cells in fed rabbits, but caused an opposite effect in both fed and fasted animals treated with estrogen. Fasting down regulated the mRNA levels for ESR1 and GnRHR. Estrogen-priming reduced the abundance for ESR1 mRNA in both fed and fasted rabbits, and that for GnRHR in fasted rabbits. Ovariectomy halved ESR1 mRNA levels independently of treatment and feeding condition, whereas increased (P < 001) that for GnRHR in estrogen-primed rabbits. In all rabbits, an LH surge occurred 30 min after GnRH injection but the lowest levels were found in intact fasted rabbits and the highest in fasted, estrogen-primed animals. The LH profile was similar in intact and OVX rabbits and neither fasting nor estrogen priming modified it. In conclusion, fasting differentially modifies the ESR1 and GnRHR expression in the pituitary, depending on the presence of gonadal hormones, indicating complex interactions between metabolic signals and ovarian steroids.